The present invention relates to polyester resins with improved rheological properties, useful in particular for applications using extrusion blow-molding and injection blow-molding techniques.
The aromatic polyester resins obtained from aromatic bicarboxylic acids and from diols are suitable for the manufacturing of fibre and film, but they do not possess sufficient melt strength to consent to their use in the manufacturing of products by extrusion blow-molding technique.
To increase the intrinsic viscosity and to improve their rheological characteristics, the resins are subjected to solid state polycondensation (SSP) reactions in the presence of polyfunctional compounds that can act as branching agents or as chain extenders. These compounds, apart from improving the polymer rheological characteristics, accelerate the kinetics of the SSP reaction.
Polyfunctional compounds, containing at least three groups capable of reacting with the terminal groups of the resin, act as branching agents. The representative compounds are polyhydric alcohol such as pentaerythritol and trimethylolpropane. Compounds preferably functioning as chain extenders are dianhydrides of aromatic tetracarboxylic acids. Pyromellitic dianhydride (PMDA) is the representative compound. The branching agents bring about in general the formation of gels that limit their use. It has been proposed in U.S. Pat. No. 4,161,579 to use the branching agents in association with the chain terminating agents in order to reduce the formation of gels. However, when utilizing this procedure in the manufacturing of bottles from polyethylene terephthalate (PET) by extrusion blow-molding, the thicker sections of the bottle wall, that is the neck, tend to present opacity which is not acceptable in the field of containers for beverages and cosmetics that have to also satisfy aesthetic prerequisites.
This inconvenience could be eliminated by extruding, in place of PET, polyethylene terephthalate copolymers at content up to ca. 15% of the unit from isophthalic acid (U.S. Pat. No. 4,234,579). The branching agents and the chain extenders are in general used in quantities superior to 0.1% in weight, preferably between 0.1 and 0.3% in weight.
From WO-A-93/234, it is known to use for applications by injection blow-molding limited quantities of polyhydric alcohol, such as pentaerythritol, comprised between 0.007 and 0.08% in moles equal to 0.005/0.057% by weight, if referring to the pentaerythritol molecular weight. This is used with the aim of limiting the formation of gels. Despite the low quantities used, the rate of the solid state polycondensation is still significant. Nevertheless, the melt strength of the resin is not increased. Higher quantities do bring an increase in the melt strength, but correspondingly increase the formation of infusions.
Furthermore, induced crystallization phenomenons are observed with the appearance of opaque areas which are not acceptable for beverage containers. The chain extenders, such as dianhydrides of aromatic dicarboxylic acids and in particular pyromellitic dianhydride, are used in quantities of at least 0.1% in weight. The kinetics of the solid state polycondensation are recognizably increased (U.S. Pat. No. 5,243,020). PET mixtures at the melt state make it possible to obtain bottles by extrusion blow-molding.
However, while PMDA type chain extenders, on one hand bring an increase in the melt strength of resin, on the other hand determine the excessive increase of the elasticity of the melted polymer. This involves a significant slowdown of the extrusion blow-molding operations because of the excessive swelling of the resin at the exit of the mold (die swell).
It has now unexpectedly been found that, by using in the solid state polycondensation reaction, quantities of dianhydride of aromatic tetracarboxylic acids having value of less than 0.1% in weight used in the processes of the known technique, it is possible to still obtain significant increases not only in the kinetics of the solid state polycondensation, but also in the melt strength of the polymer such as to consent parison stability, and at the same time not causing a too high level in die swell values.
It has also been found, and this represents an additional aspect of the present invention, that the use of limited quantities of the aromatic tetracarboxylic acid dianhydrides allows significant improvement in the mechanical characteristics (compression and burst resistance, etc.) and barrier properties of the containers obtained by injection blow-molding techniques. The average crystallinity of the PMDA modified PET bottle walls is higher than that of the comparative bottles without PMDA. In the case of bottles and film obtained from PET modified with 4.5% by mols of isophthalic acid and 7.5% by mols of 2,6-naphthalene dicarboxylic acid and 0.05% by weight of PMDA, a crystallinity of up to 60% in the side walls of the bottles and in the film by heat setting the bottles and the film at temperature from 160xc2x0 to 215xc2x0 C.
The increase in the mechanical and barrier properties allows the manufacturing of containers with thinner walls which use lower quantities of resin. The dianhydride quantities utilized are less than 0.1% in weight and comprise between 0.1 and 0.015%, preferably between 0.05 and 0.02% of the resin weight. The dianhydrides are added in the preparation phase of the resin by melt polycondensation as well as to the polymer obtained after this phase. The intrinsic viscosity of the resin after the melt state polycondensation is generally inferior to 0.7 dl/g; it is brought to a desirable level comprised between 0.7 and 1.5 dl/g by means of solid state polycondensation.
The preferred dianhydride is pyromellitic dianhydride. Other anhydrides used are dianhydrides: of the following acids: 2,2 bis (3,4 dicarboxyphenyl) propane; 3,3xe2x80x2, 4,4xe2x80x2-benzophenone-tetracarboxylic, bis (3,4-dicarboxyphenyl) ether; 2,2-bis (3,4 dicarboxyphenyl) hexafluoro propane, 2, 3, 6, 7-naphthalene tetracarboxylic; 1, 2, 5, 6 naphthalene tetracarboxylic; bis (3, 4-dicarboxyphenyl) sulfoxide.
Belonging to this dianhydride category also are the addition compounds containing two groups of phthalic anhydride obtained by the reaction of 2 moles of dianhydride of aromatic tetracarboxylic acid with one mole alkylene glycol or polyalkylene glycol in particular ethylene glycol and polyethylene glycol or of other composition containing at least two hydroxylic terminal groups.
The solid state polycondensation temperature generally is between 150xc2x0 and 220xc2x0 C. The preferred temperatures are between 180xc2x0 and 220xc2x0 C.
In the case of copolyesters in which part of the terephthalic acid units are substituted by units deriving from other aromatic dicarboxylic acids, for example isophthalic acid, the reaction temperature at the solid state could be comprised in a range inferior to that mentioned above. The dianhydrides are present in the final polymer, after SSP, in combined form. The resins subjected to the solid state polycondensation reaction are obtained according to known procedures by polycondensation of a diol containing from 2 to 12 carbon atoms with an aromatic bicarboxylic acid or its own ester. Representative diols are ethylene glycol, 1-3 propylene glycol, butylene glycol, dimethylolcyclohexane. Terephthalic acid, isophthalic and naphthalene dicarboxylic acids (2,6 and 2,7-naphthalene dicarboxylic) are preferred. The preferred resins include polyethylene terephthalate and its copolymers in which part of the units of the terephthalic acids are substituted by units from one or more dicarboxylic acids of which isophthalic acid and/or naphthalene dicarboxylic acids can be used in quantities of up to ca. 20% in moles on terephthalic acid. The most preferred resins are PET-ter polymers with 2 to 10% isophthalic acid and with 2 to 10% naphthalene dicarboxylic acids.
The resins thus obtained are subjected to SSP treatment. Before this treatment, the resins undergo a crystallization treatment, according to known procedures so as to eventually reach crystallinity level values sufficiently high in order to avoid phenomenons of packing and/or adhesion of polymer granules on the walls of the SSP reactor.
Equipment and procedures used are described for example in EP-A-222 714, EP-A-373 684, U.S. Pat. No. 4,064,122, U.S. Pat. No. 4,161,578 and in EP-A-712 703, the descriptions of which are herewith incorporated by reference.
The SSp treatment is also carried out in the equipment according to known techniques. For example, the washing gases used in the process are purified according to the process of WO-A-95 02 446, whose description is also incorporated by reference. As already indicated, the dianhydride of the tetracarboxylic acid is incorporated both during the preparation phase of the resin by the melt state polycondensation as well as to the resin already formed. In the first case, it is dissolved in the glycol used for the reaction; in the second case, it is added in the extruder to the melted resin.
The second alternative is preferred. A counterrotating non-intermeshing twin screw extruder is preferably used with average residence time in the extruder inferior to ca. 100 seconds. The granules thus obtained are subjected to crystallization and then solid state polycondensation.
Additives normally used for polyester resins such as stabilizers, dyes, nucleants and others can be added to the resin.
The following examples are provided to illustrate but not to limit the invention.